1. Field of the Invention
This invention relates to an improvement in the structural arrangement of an electromagnetically operated shutter for a camera.
2. Description of the Prior Art
Generally, an electromagnetically operated shutter uses an electromagnetic force for driving shutter blades. The electromagnetic force F is expressed as F=BIL and is proportional to magnetic flux density (B), a current (I) and coil length (L) respectively. In order that every electromagnetically operated shutter to be manufactured have a uniform driving force, therefore, the magnetic flux density, the current and the coil length must be kept in an unvaried condition. As for the coil length, it is possible to wind or pattern a coil to an undeviating uniform length so that coils of uniform length can be always obtained. It is also possible to obtain an undeviating value of current through an electric circuit. However, it has been impossible to obtain a uniform degree of magnetic flux density for every shutter manufactured because of a problem relative to positioning a permanent magnet. In other words, in the conventional electromagnetically operated shutter, a permanent magnet is arranged without placing it in a fixed position on a magnetic base plate used for forming a magnetic circuit. Therefore, the permanent magnet and the coil of one shutter have differed in position from those of another shutter. Besides, a spacer or the like inserted in a space between the permanent magnet and a yoke to form "a gap of a magnetic circuit" has made it difficult to attain dimensional precision. This difficulty has furthered the unevenness of shutters in respect to magnetic flux density and also has complicated assembling work.
Further, to better the dynamic characteristic of a shutter, it is advantageous to have a greater driving force. In view of this, a uniform high magnetic flux density has been obtained in practice by using a stronger permanent magnet. The use of a strong permanent magnet, however, presents a problem in that the magnetic flux might leak to some parts outside of the magnetic circuit. Such a magnetic flux leakage tends to magnetically affect other parts of the camera such as a shutter control circuit, and thus tends to cause troubles in shutter control, etc.
The degree of overlapping of shutter blades hardly can be made uniform among shutters due to manufacturing errors, etc. Then, the uneven overlapping of shutter blades makes the dynamic characteristic, in the initial stage of movement, of one shutter different from that of another. This results in an exposure error. It is, therefore, necessary to have the overlapping of the shutter blades adjusted to a uniform degree by means of an adjusting member. Then, with such an adjusting member used for an electromagnetically operated shutter of the above stated type, it is necessary to have the adjusting member disposed on the base plate which forms a magnetic circuit. The adjusting member is to be adjusted with some tool such as a screw driver upon completion of shutter assembly. If the adjusting tool is made of a magnetic material, there would take place an attraction between the tool and the base plate which forms the magnetic circuit. The attraction would hinder the adjusting work. Then, if the work must be done with a tool made of a non-magnetic material, it necessitates selection of such a tool solely for the adjusting work on this part while other parts of a camera are assembled and adjusted with tools made of a magnetic material. This complicates the work. Further, even if a tool made of any material is used as desired, magnetic dust sticking to the tool would still be attracted to the base plate around the adjusting member during the adjusting work thereon, and then might remain there after adjustment to hinder the rotation of the rotor or the movement of the shutter blades.